Process of making an improved filter media

ABSTRACT

A method for preparing an improved filter medium of enhanced electrokinetic capture potential for contaminated liquids having suspended therein electro-negative particles of about 0.1 to 0.7 micron diameter. The method comprises forming a filter medium from a dispersion of negatively charged particulate material for filtration and a sufficient amount of fibers to form a matrix of self-bonding fibers. The surface of the particulate has been modified with a melamine formaldehyde cationic colloid.

RELATED APPLICATION

This application is a division of U.S. Ser. No. 898,578 filed Dec. 23,1977 now abandoned, which is a continuation of U.S. Ser. No. 709,564filed July 28, 1976 now abandoned, which in turn is a division of U.S.Ser. No. 598,097, filed July 22, 1975 and is now U.S. Pat. No. 4,007,113which is a continuation-in-part of U.S. Ser. No. 358,822, filed May 9,1973, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a new process for treatment of negativelycharged high surface area particulate filter material to improve thefiltration efficiency for small particle, especially submicronparticles, and the improved filter media obtained thereby.

The filtration of fine particle size contaminants from liquids has beenaccomplished by the use of various porous filter media through which thecontaminated liquid is passed. With particulate filter materials, thefilter media often takes the form of a porous cake or bed of theparticulate material deposited on a porous support or substrate. Tofunction as a filter, the media must allow the fluid, commonly water,through while holding back the particulate contaminant. This holdingback of the particulate contaminant is accomplished by virtue of theoperation, within the porous media, of one or both of two distinctlydifferent filtration mechanisms, namely (1) mechanical straining and (2)electrokinetic particle capture. In mechanical straining, a particle isremoved from the fluid stream by physical entrapment when it attempts topass through a pore smaller than itself. In the case of theelectrokinetic capture mechanism, the particle collides with a surfacewithin the porous material and is retained on the surface by theattractive short range van der Wall's type forces.

In those particulate filter media that depend upon mechanical strainingto hold back particulate contaminants, it is necessary that the poresize of the filter medium be smaller than the particles size of thecontaminants that are to be removed from the fluid. If it is desired toremove suspended solids of a fine particle size with a particulatefilter medium that functions by mechanical straining, the cake or bedformed by the filter material need have correspondingly small pores.Such a filter medium normally exhibits low flow rates and a tendency toclog rapidly.

In those filter media that function by virtue of the electrokineticparticle capture mechanism, it is necessary that the filter medium havesuch a small pore size. The ability to achieve the required removal ofsuspended particulate contaminants with a filter medium of significantlylarger pore size is attractive inasmuch as it allows higher flow ratesand reduces the tendency of the filter medium to clog rapidly. Theability of a filter medium to hold back suspended solids of a fineparticle size by electrokinetic particle capture depends, to asignificant extent, on the surface properties, particularly the surfacecharge, of both the filter material of which the medium is composed andthe suspended solids. It is not feasible to measure the charge on asurface directly. Instead, various indirect techniques, such aselectrophoretic mobility, streaming potential, etc., are used todetermine the zeta potential, i.e., the electric potential excess of thesurface, and the surrounding fluid to the hydrodynamic shear plane, overthe bulk potential of the fluid. Inasmuch as surface charge can only bequantified in terms of the zeta potential, all further characterizationsof surface charge will be in terms of the zeta potential. While the zetapotential exhibited by a surface normally depends upon the compositionof the filter material, it may be modified by other materials thatbecome adsorbed or chemically bonded to the surface of the filtermaterial.

The effect of zeta potential on the electrokinetic capture mechanism isassociated with the ability of the suspended solid contaminant particleto come into contact with a surface within the proous filter medium. Inorder for such contact to occur, it is necessary that either thecontaminant particle or filter material surface possess a zero zetapotential, or that the surface have an opposite zeta potential from thatof the contaminant particle. If the contaminant particle and the filtermaterial surface have like zeta potentials, there will be a repulsiveeffect that interferes with the particle's ability to come into contactwith the surface. Once in contact with the surface, the particle will beretained by short range van der Waal's forces, which are alwaysattractive. In those situations where electrokinetic capture does occur,the filtration performance is enhanced by the availability of a highfilter material surface area within the filter medium. The reason forthat is that, as the oppositely charged particles deposit on the surfaceof the filter material, the deposited particles tend to modify the zetapotential of the surface and, eventually, the surface will develop alike zeta potential, effectively inhibiting any additional deposition.The availability of a high opposite charge and a high surface area thusextend the functional life of the filter medium. The particle size, andthe geometry, porosity and depth of the filter medium also affect thelife and filtration efficiency.

Asbestos fiber has long been used for filtration of fine or very finesolids and the use thereof has been welldocumented in the literature,e.g. Proceedings of the Filtration Society, Filter Sheets and SheetFiltration by Geoffrey Osgood, published in Filtration and Separation,July/August 1967, pp. 327-337 (A paper originally presented at theFiltration Society Meeting, London, Apr. 4, 1967), also, Proceedings ofthe Filtration Society, Asbestos Filter Sheets by D. McLean Wyllie,published in Filtration and Separation, /March/April 1973, pp. 175-178(A paper originally presented at the second joint AICHE-FiltrationSociety Symposium, Minneapolis, Minn., Aug. 27-30, 1972). The highfiltration efficiency of asbestos fibers is attributable not only tomechanical straining effects but also to the fineness and high surfaceareas of the fibers, which, in conjunction with the positive zetapotential exhibited by asbestos, result in a highly efficientelectrokinetic capture of negatively charged contaminant particles.Attempts to duplicate the filtration efficiency of asbestos in othermaterials by duplicating the physical state (size, shape, etc.) ofasbestos fibers have, up to the present, not been effective. Forexample, duplication of asbestos filter material with glass fibers, ordiatomaceous earth, has not provided filtration efficiency evenapproaching that of asbestos. No other filter material has a positivezeta potential of the same order as asbestos fiber.

In the past, improvement in the filtration efficiency of filter materialhas taken several approaches. Improved filtration performances have beenrealized by chemical treatment to alter the surface properties and,thereby, the size of the suspended solids (i.e., flocculation) toimprove the straining effects. (See U.S. Pat. Nos. 3,131,144; 3,227,650;3,325,492; 3,297,106; 3,542,674; 3,562,154 or 3,668,184). Improvedfilter performance is also realized by altering the surface propertiesof the filter material. (See U.S. Pat. Nos. 2,040,818; 2,036,258;2,797,163 or 2,971,907).

Development of the art has favored the application of specific highlycharged polyelectrolyte materials. Thus, U.S. Pat. Nos. 3,352,424 and3,242,073 describe the coating of filter materials with organicpolyelectrolytes, e.g. polyalkylenimines. Baumann et al, in"Polyelectrolyte Coatings for Filter Media" Proceedings of theFiltration Society: Filtration and Separation (Nov./Dec. 1970) pp.652-690 describe additional research.

These polyelectrolyte materials are unfortunately of relativelysophisticated structure, high molecular weight, high charge density andconcomitantly have high manufacturing cost. Accordingly, advantage isseen in the provision of a cheaper but efficacious substitute.

An improvement in liquid filtration as for food and beveragecompositions, pharmaceuticals, or in water purification and the like,especially for submicron negatively charged contaminants has now beenachieved by modifying the surface of negatively charged particulate highsurface area filter materials with a cationic melamine formaldehydecolloid. Surprisingly, the improvement is afforded by treatment with thecolloid form of a melamine formaldehyde resin despite its low molecularweight and low specific charge. The resin is applied, as hereinafterdescribed more particularly, to the particulate filter material, and theporous filter medium in the form of a filter bed or cake is subsequentlyformed dynamically in a manner well-known to the art, as by vacuumtechniques.

Melamine-formaldehyde resins including the cationic colloid arewell-known and these resins have commonly been used in the paperindustry to impart water resistance, i.e. wet-strength, as shown in U.S.Pat. No. 2,563,897. Melamine-formaldehyde colloids are formed fromsolutions of a melamine monomer acid addition salt by condensation ofabout 20 units of the monomer with elimination of water to form cationiccolloid particles. During formation, some of the acid of the monomeraddition salt is liberated and the progress of condensation can befollowed by measuring the drop in pH. The chemistry ofmelamine-formaldehyde colloids is described in numerous literaturearticles, e.g. Chapter 2, Melamine Formaldehyde, C. S. Maxwell, WetStrength in Paper and Paper Board, Tappi Monograph Series #29, 1965 andAmino Resins, John J. Blair, pp. 19-25, Reinhold Publishing Corp., NewYork, 1959, and the specific colloids of the invention may be preparedin accordance with these disclosures or U.S. Pat. Nos. 2,345,543;2,559,220 or related U.S. Pat. Nos. 2,485,079-80. The compoundsgenerally designated melamine-formaldehyde embrace the methylol melaminemonomers in which there are from 1 to 6 methylol substituents, the mostcommonly employed being the di- or trimethylol compounds.

High surface area particulate filter materials are well-known in the artand include such recognized materials as diatomaceous earth, perlite,sand, etc. These materials are characterized by either thefinely-divided state in which they exist, or a fine internal porosity,either of which provides a high surface area per unit volume. Thenon-porous particulate solids such as perlite and sand are characterizedby particles of a size small in comparison to the normally used depth ofthe filter bed, whereas diatomaceous earth is additionally characterizedby a fine internal pore structure. These high surface area particulatefilter materials tend to exhibit a negative zeta potential.Consequently, when such material is used for filtration of fine particlesuspended solids, there is no particle removal by electrokinetic capturebecause most suspended solids are also negatively charged. Thus, theonly filtering effect attainable with these negatively charged filtermaterials is the so-called straining effect described hereinbefore.Thus, the efficiency of such filter materials with fine particlesuspended solids is limited and never approaches the filtrationefficiency of positively charged filter materials known to be useful forthis purpose, e.g. asbestos fibers.

SUMMARY OF THE INVENTION

The present invention is concerned with overcoming the deficiencies ofthe negatively charged, high surface area particulate filter materialsin an economical manner to permit use of these materials in filter mediafor filtration of finely-divided suspended solids. Essentially, thepresent invention utilizing a melamine-formaldehyde colloid provides aprocess which at least reduces the negative charge on filter materialsof the type described and thus enhances electrokinetic capture of fineparticle suspended solids. The colloid particles appear to adhere to thesurface of the filter material producing highly positively charged siteson the surface so that filter media composed of particulate filtermaterials so treated are capable of removing, by electrokinetic capturefine particle suspended solids which without such modification, wouldnot be removed by the filter medium. Filtration efficiencydeterminations indicate that modified filter media can produce the sameorder of efficiency as asbestos fibers. In view of the present knowledgeof filtration mechanisms and the nature of asbestos function, thesecomparative data suggest that the modified filter media have a positivecharge similar in nature to that of asbestos and that the enhancedfiltration efficiency is attributable to this charge modification. Itappears then that the deposited melamine-formaldehyde colloid particlessurprisingly must retain their cationic properties after adsorption anddrying if in fact the mechanism of filtration occurs as theoreticallypostulated. While not wishing to be bound by any theoretical explanationof the phenomenon observed, the applicant offers the foregoing as anexplanation of the results observed, which are surprisingly and mostunexpected since the melamine-formaldehyde colloids would not beexpected to retain their cationic properties after their deposition ontothe fiber surface and subsequent drying and this phenomenon has not beenrecognized in the prior art.

The process of this invention is accomplished by merely contacting themelamine-formaldehyde colloid with the selected filter material,conveniently in a suitable medium. The colloid dispersions are normallyand conveniently prepared in aqueous media which is also suitable forthe modification of the filter material. In a simple form of the presentprocess, the filter material is added to an aqueous dispersion of thecolloid particles and mixed by any convenient means to assurecommingling of the respective solids. In a very short period of time,the colloid deposits on the filter material, usually within severalminutes, and the mixture can be further processed as desired to separateand dry the modified filter material. Alternative mixing sequences, andalternative treatment techniques such as spraying or blending areequally suitable as means for treating the filter material with themelamine formaldehyde colloid. If desired, the filter material can befurther treated while in the aqueous mixture or in the final driedcondition by addition of further materials, e.g. cellulose fibers whichcan serve as a self-bonding matrix to form a sheet type filter medium.

The process conditions are not critical as long as the colloid ispermitted to contact the filter material. The amounts of the dispersionmedium, e.g. water, do not seem to be critical. Even when large volumesof water are used, i.e. a dilute medium with only minimum amounts ofcolloid (e.g. below 5% by weight of filter material), and filtermaterial (e.g. about 1-2% by weight of liquid) modification of thefilter material occurs. The dispersing medium employed should be polarto permit the deposition to proceed efficiently. The time required formodification does not appear critical since deposition reaches a highpercentage of final equilibrium values almost instantly with about 0.5to about 6 minutes being adequate for most purposes. Of course, longerperiods of exposure, e.g. up to several hours, can be used to assurerelatively complete deposition. A period of about 15 minutes up to twohours is typical.

The amount of colloid added to the filter material is not critical butis merely a matter of functionality. For example, a high surface areafilter material may require more resin for optimum filtration than oneof lower surface area. Nevertheless as the resin is adsorbed onto thesurface area of the material, the filtration efficiency is enhanced, sothat even small amounts of the colloid are effective.

The melamine-formaldehyde used to produce the colloid in the presentprocess can readily be obtained from commercial sources. Most commonly,a di- or trimethylolmelamine is used and is generally preferred. Themonomethylol melamines do not appear to be as effective while thetetra-, penta- and hexa-methylolmelamines do not appear to provide anyadvantage over the di- or trimethylol derivative and are more expensive.If desired, these melamine derivatives can be prepared as described inthe scientific literature.

The filter material is, of course, readily attainable from commercialsources and may be prepared by methods described in the literature.Thus, diatomaceous earth filter aid is obtainable from Johns-Manville,Celite Division, in a variety of particle sizes. Similarly, perlite ofvarious dimensions is available from Dicalite, Division, Grefco Inc.

For the purpose of the present invention, any high surface area,particulate filter material which exhibits a negative zeta potential canbe modified by the present process to improve the filtrationperformance. In the present disclosure and the appended claims, theexpression "high surface area" is intended to mean fine particle sizeporous and non-porous particulate materials such as diatomaceous earthor perlite with high specific surface areas e.g. in excess of 1 m² /gm.or non-porous particulate materials, such as glass spheres and sand, inwhich the diameter of the particulate is small in comparison to thedepth of the filter bed formed, (e.g. sand particles range up to severalhundred microns and glass up to 500 microns in diameter, but filter bedsof these materials are measured in feet, such as an eight foot depthsand bed); the expression "negative filter material" means a filtermaterial which exhibits a negative zeta potential. In every instance,reference to diameter is intended to refer to average diameter as iscommon practice in referring to fine particle dimensions.

BRIEF DESCRIPTION OF THE DRAWING

The only FIGURE in this application depicts the filtration efficiencyfor treated (A) and untreated (B) filter material of a test described indetail in Example I.

DETAILED DESCRIPTION OF THE INVENTION

A preferred filter material is diatomaceous earth. Other suitable filtermaterials include non-porous particulate solids, such as perlite orsand; or mixtures in all proportions of materials such as diatomaceousearth/perlite. Perlite containing media are preferred where bulk densityis a consideration.

Low molecular weight (1000-4000) melamine-formaldehyde resin colloidsare preferred since they are readily prepared or available and providestable colloids with substantial shelflife in aqueous suspension.Colloids prepared from di- and tri-methylolmelamines are preferred. Thisinvention contemplates the use of other substitutedmelamine-formaldehyde resins which form cationic colloids similar tothose produced with di- or trimethylolmelamine, but the di- andtrimethylolmelamines are preferred because they are commerciallyavailable and economical, and readily form colloids. Higher methylolcontent does not seem to be any more efficient than thetrimethylolmelamine resins and, because they are more costly, the tetrato hexamethylolmelamines are less attractive for use.

The liquid medium for the present process is a polar medium,advantageously an aqueous medium as mentioned hereinbefore. Water is thesimplest, most adequate and most economical and therefore is preferred.

In the modification process, the colloid and the filter material aremerely added to water and then subjected to mixing to provide intimatecontact between the respective solids. For most purposes, it issufficient to stir the mixture for short periods of time to permitmixing of the respective solids and then allow the suspended solids tosettle out. Usually, the colloid is added to the reaction mixture in theform of an aqueous dispersion of from about 3% to about 50% by weight,although from about 5% to about 20% by weight is preferred. Typically5-7% solids is used with Parez 607 colloid. The weight of colloid basedupon filter material in the dispersion ranges from the minimum to effectcharge modification up to about 50% preferably about 10%, and up toabout 40%, with optimum results seen for selected materials in the 5 to25% range. As hereinbefore mentioned, the amount of water used as themedium is not critical but sufficient water should be present to permitthe necessary contact between the colloid and filter material.

After mixing, the surface-modified filter material can be separated fromthe reaction mixture by any method which is conventional, e.g.decantation followed by filtration through a suitable filter. Theseparated filter material can then be dried, washed or further treatedto produce the final filter form according to recognized procedures.Where desired, a matrix of self-bonding fibers may be used to formfilter sheet. The self-bonding fibers can be added directly to thefilter aid dispersion and, after further mixing, the entire mass canthen be processed into filter sheet by methods well-known to the art.The preferred self-bonding fiber for the matrix is cellulose, which iscommonly used in filter sheet production. The usual amount of cellulosefibers used as matrix lies in the range of from about 50% to about 90%by weight of the filter sheet.

The filtration efficiency realized with the surfacemodified filtermaterials of this invention are illustrated in the enclosed FIGURE, aplot of percentage efficiency of filtration versus the particle size ofsuspended solids being filtered, for a diatomaceous earth treated withmelamine-formaldehyde colloid (curve `A`) compared with untreated D.E.(curve `B`). The FIGURE clearly illustrates the unexpectedly higherefficiency of surface charge modified filter in removal of suspendedsolids of submicron dimension, e.g. less than about 0.7 micron indiameter.

In the following examples further illustrating the invention the testprocedures, techniques and materials utilized are as follows:

1. Preparation of Colloid

A. Materials (for 10 gallons):

    ______________________________________                                        Water, 70 ± 20° F.                                                                        8.8    gallons                                          Muriatic Acid, 30° Be                                                                        0.4    gallons                                          "Parez" Resin 607     10.0   pounds                                           ______________________________________                                    

Add resin to acid-water mixture using agitation until complete solutionis obtained (approx, 30 minutes as indicated by colorless, water-clearsolution). An aging period of at least three hours is required to allowformation of colloid as indicated by formation of distinct bluish haze.For better stability in storage, the colloid dispersion is diluted tosix percent solids.

B. Improved performance for Parez 607 systems may be secured byutilizing reagent grade hydrochloric acid, at an acid ratio (molesHcl/moles resin) of 0.60 to 0.65.

Preparation A was employed for Example I, and preparation B for theremaining Examples.

2. Treatment

Particulate filter material, at a 2.5% consistency, is slurried inwater, and the colloid dispersion added (6% solids) to provide a filtermaterial/colloid weight ratio of 10:1. Contact time is at least 15minutes whereupon the treated material is isolated, drained and dried at250° F. for 30 minutes.

3. Test Contaminant

Test contaminant except as otherwise noted consisted of monodispersepolystyrene latices (Dow Diagnostics) of the following sizes:

0.109 microns

0.234 microns

0.357 microns

0.481 microns

0.794 microns

The particular latex used in a test is mixed with double glass distilledwater to produce a dispersion with a turbidity of 100 FTU (FormalinTurbidity Units, also referred to as the equivalent, NTU orNephalometric Turbidity Units) as measured on a Hach Model 2100ATurbidimeter. The pH of the test dispersion is 6.5-7.0.

4. Test Technique (Efficiency vs. Particle Size)

The treated particulate filter material is slurried in 100 ml of waterand filtered through the porous fritted glass holder base in a Millipore47 mm vacuum filter holder, until a 1/4" thick cake is formed (typically13/8" diameter). 100 ml of the prepared latex contaminant dispersion isfiltered through the cake using a 23 in. Hg vacuum. The turbidity of thefilter effluent is measured using the Hach Turbidimeter. Filtrationefficiency is determined from the formula: ##EQU1## Untreatedparticulate filter material is also tested for comparison.

EXAMPLE I

2.5 grams of commercially available diatomaceous earth filter aid (J. M.Celite 545 relative flow rate 2160; 11.7% retained on 150 mesh screen)is slurried into 100 ml of water. To this is added 8.33 grams of the 6%resin colloid dispersion prepared as described above (Preparation A).The resulting slurry is agitated for 15 minutes.

The treated diatomaceous earth is dewatered by filtration using #54Whitman filter paper, and then dried in an oven at 250° F. for 30minutes. The resulting material is easily redispersed into water aftertreatment.

The filtration efficiency for treated (`A`) and untreated (`B`) filtermaterials is as shown in the accompanying FIGURE, illustrating the highefficiency of the surface charge modified material in removal ofsuspended solids less than about 0.7 micron in diameter.

EXAMPLE II

A. Treated diatomaceous earth samples of different relative flow rating(Johns-Manville:indirect measure of particle size) were tested forfiltration efficiency and compared to untreated control samples all inaccordance with the previously desired test procedure giving thefollowing results:

                                      TABLE I                                     __________________________________________________________________________              % Retained                                                                           Relative-flow rating                                                                          Filtration Efficiency %                      D.E.      on 150 mesh                                                                          (Johns-Manville)                                                                              .109 micron                                                                         .234 micron                                                                         .357 micron                      __________________________________________________________________________    Filter Cel                                                                              0.6    100       Untreated                                                                           79    88    --                               (natural)                  Treated                                                                             99.8+ 99.9+ --                               Celite 505                                                                              --     135       Untreated                                                                           32    44    --                               (calcined)                 Treated                                                                             99.8+ 99.9+                                  Standard Super Cel                                                                      2.5    200       Untreated                                                                           30    --    --                               (calcined)                 Treated                                                                             99.6  99.7  --                               Celite 512                                                                              4.4    300       Untreated                                                                           23    55    73                               (calcined)                 Treated                                                                             96.4  99.8  99.7                             Hyflo Super Cel                                                                         5.0    500       Untreated                                                                           10    41    63                               (flux-calcined)            Treated                                                                             97.7  99.4  99.6                             Celite 501                                                                              7.5    750       Untreated                                                                           14    33    41                               (flux-calcined)            Treated                                                                             90.7  99.7  740                              Celite 503                                                                              8.8    900       Untreated                                                                           3     47    64                               (flux-calcined)            Treated                                                                             84.0  99.7  99.9                             Celite 535                                                                              9.1    1350      Untreated                                                                           --    30    47                               (flux-calcined)            Treated                                                                             --    94.3  99.3                             Celite 545                                                                              11.7   2160      Untreated                                                                           --    34    54                               (flux-calcined)            Treated                                                                             --    97.3  99.5                             Celite 560                                                                              48.    7500      Untreated                                                                           --    --    45                               (flux-calcined)            Treated                                                                             --    --    97.7                             __________________________________________________________________________

The efficiency of the untreated D.E. is seen to drop sharply for fineparticle size contaminants as its own dimension increases, a strainingphenomena reflecting the tightness of the cake structure with the finefilter aid types. However, treated materias evidence an improvement inperformance in each case over the range. Similar results are obtainedwith Speed Plus (a flux-calcined D.E. of 9.1 micron mean particle size).Speedflow (calcined, 6.0 micron) and Dicalite 215 (calcined, 2.7micron); and filtration improvement noted with drained but not driedsurface charge modified materials.

B. As a comparison, D.E. samples were treated with a 6% solution of awater soluble melamine-formaldehyde resin (Cymel 481), drained and driedat 250° F. for 30 minutes. The filtration efficiencies using the sametest procedure, relative to a control (untreated) D.E. are reported inthe following:

                  TABLE II                                                        ______________________________________                                                   Filtration Efficiency %                                                         .109    .234    .357  .481  .794                                              mi-     mi-     mi-   mi-   mi-                                               crons   crons   crons crons crons                                ______________________________________                                        Celite Untreated 17      32    80    72    96                                 503    Treated                                                                       Cymel 481 0       62    89    93    92                                 Celite Untreated 6       17    17    58    94                                 545    Treated                                                                       Cymel 481 0       13    26    28    53                                 ______________________________________                                    

In the region (lower particle size contaminant) where discrimination ofresults is meaningful, the water soluble resin becomes less effectivewith reducing particle size contaminant (especially with the largerparticle size D.E.) as distinct from the colloidal resin.

EXAMPLE III

Treated Dicalite Perlite grade 416 (mean particle size 3.9 microns,surface area 3.65 m² /g) samples were prepared with Parez 607 colloid,and a solution of Cymel 481, (water-soluble non-colloidalmelamine-formaldehyde) respectively, at an equal solids weight bases(10:1 ratio of filter material to colloid in the respectivedispersions).

The resulting samples, along with an untreated control were tested inaccordance with the previously described procedures, giving thefollowing results.

                  TABLE III                                                       ______________________________________                                                     Filtration Efficiency %                                                                   Perlite 416                                                                             Perlite 416                                Test Contaminant                                                                             Control   Treated   Treated                                    Dow Diagnostics Latex,                                                                       Perlite 416                                                                             Parez 607 Cymel 481                                  100 NTU        Untreated colloid   non-colloid                                ______________________________________                                        .109 micron     0        86         0                                         .234 micron    77        98        33                                         .357 micron    82        97        79                                         .481 micron    83        98        90                                         .794 micron    88        99        91                                         ______________________________________                                    

The remarkably high efficiency of the colloid treated perlite isapparent, especially with the lower particle size contaminants.

Similar results are obtained with Perlite 416 (3.9 micron), Perlite 426(4.2 micron) and Perlite 4106 (10 microns); and filtration improvementnoted with drained but not dried surface charge modified materials.

EXAMPLE IV

A. Coarse washed sand (40% 880 to 1100 micron, 30% 440 to 880 micron,and 30% 110 to 440 micron) was dispersed in water to form a slurry, theslurry was disposed in a columnar filter housing and drained and dried,to form a filter bed of 5/8 inch diameter 23 inches in depth. A similarsand slurry was treated in standard manner with Parez 607 colloid.

The respective beds were then competitively tested for filtrationefficiency as described above, with the following results.

                  TABLE IV                                                        ______________________________________                                                                     Filtration                                       Test Contaminant             Efficiency, %                                    Dow Diagnostics Latex,                                                                      Control        Treated Sand                                     100 NTU       (Untreated Sand)                                                                             (607 Colloid)                                    ______________________________________                                        0.109 micron  0 = no measurable                                                                            72                                                             reduction                                                       0.234 micron  0 = no measurable                                                                            67                                                             reduction                                                       0.357 micron  0 = no measurable                                                                            75                                                             reduction                                                       0.481 micron  0 = no measurable                                                                            76                                                             reduction                                                       0.794 micron  10             91                                               ______________________________________                                    

B. Further testing was carried out with water soluble non-colloidalmelamine-formaldehyde resin (Cymel 481) on an equal weight basis, and asand bed formed in the same manner. The resin aided filtration through abonding action, but efficiencies for the respective test contaminantspecies were only 42, 56, 59, 72 and 83. The masking effect of strainingtype filtration is seen with the larger particle size contaminants withmarked improvement evidenced for the colloid with the smaller particlesizes.

EXAMPLE V

In a further series of experiments, the level of applied colloid wasvaried to determine the effect upon effluent turbidity, utilizing a testsolution of water and polystyrene latex contaminant (0.109 micron) withinlet turbidity established at 100 FTU. The results are tabulated asfollows:

    ______________________________________                                                  Colloid Percent                                                                          Filtration Efficiency, %                                 ______________________________________                                        Filter Cel (D.E.)                                                                         0            79.0                                                             5            97.7                                                             10           99.8+                                                            20           99.5+                                                Celite 505  0            32.0                                                             5            97.9                                                             10           99.8+                                                            20           99.4+                                                Std. Super Cel                                                                            0            30                                                               5            93.3                                                             10           98.6                                                             20           96.0                                                 ______________________________________                                    

Optimized performance is evidenced in the region of 10% by weight ofcolloid.

The foregoing Examples illustrate the remarkable improvement infiltration performance achieved with the surface modification of highsurface area filter materials utilizing melamine-formaldehyde colloids.Surprisingly, little effect is seen on normal cellulose fiber asemployed in filter paper; the following comparative Example isrepresentative.

EXAMPLE VI

Cellulose (Coho kraft wood pulp) was slurried with a 6% Parez 607 acidcolloid dispersion to provide 10% by weight of colloid relative to thecellulose, and a sheet was formed in conventional manner. (300 gm, 27×27inch sheet)

Performance of treated and untreated sheet, formed in the same manner,was determined in a membrane protection test. A test solution of watercontaminated with a polydisperse acrylic latex (Hyplar manufactured byGrumbacher) produced by emulsion polymerization and comprising colloidalpolymer particles ranging from 0.05 to 1.0 microns (50 FTU) was pumpedthrough the test filter media and a membrane (47 mm, 0.22 micron) inseries at a flow rate of 225 ml/min, and differential pressure with timerecorded.

Neither treated nor untreated filter media provided membrane protection,failure occurring within one minute.

Similarly, asbestos-cellulose filter media show no improvement infiltration when treated with the colloid, because deposition occurs onlyon the cellulose fibers (asbestos-fibers, being cationic, will notaccept the cationic colloid).

Filter media in accordance with the invention, prepared in a formadapted for use as filter cartridges or elements, i.e. as discs, sheets,or cylinders may be employed alone or in combination with untreatedfilter media to treat pharmaceuticals such as antibiotics, salinesolutions, dextrose solutions, vaccines, blood plasma, serums, sterilewater or eye washes; beverages, such as cordials, gin, vodka, beer,scotch, whisky, sweet and dry wines, champagne or brandy; cosmetics suchas mouthwash, perfume, shampoo, hair tonic, face cream or shavinglotion; food products such as vinegar, vegetable oils, extracts, syrups,fruit juices, make-up water or cooking oils; chemicals such asantiseptics, insecticides, photographic solutions, electroplatingsolutions, cleaning compounds, solvent purification and lubricatingoils; and the like for retention of submicronic particles, removal ofbacterial contaminants and resolution of colloidal hazes.

In the foregoing examples, the melamine-formaldehyde resin used isidentified as Parez Resin 607 which is an unfilled, spray-driedtrimethylolmelamine which is normally used in the paper industry for itswet strength properties. Similar results are obtained with Cymel 409, adimethylolmelamine product, when it is used in place of Parez Resin 607.Both resins are available from American Cyanamid.

What is claimed is:
 1. A method for preparing an improved filter mediumof enhanced electrokinetic capture potential for contaminated liquidshaving suspended electronegative particles of about 0.1 to 0.7 microndiameter comprising:forming a filter medium from a dispersion ofnegatively charged particulate material for filtration and a sufficientamount of fibers to form a matrix of self-bonding fibers, the surface ofthe particulate which has been modified with a melamine-formaldehydecationic colloid.
 2. The method of claim 1, wherein said particulatematerial is selected from the group consisting of diatomaceous earth,perlite, sand, glass spheres, or mixtures thereof.
 3. The method ofclaim 1, wherein said particulate material is diatomaceous earth.
 4. Themethod of claim 1, wherein said particulate material is perlite.
 5. Amethod for preparing an improved filter medium of enhancedelectrokinetic capture potential for contaminated liquids havingsuspended electronegative particles of about 0.1 to 0.7 micron diametercomprising:modifying the surface of negatively charged particulatematerial with a melamine formaldehyde cationic colloid, and thereafterforming a filter medium from a dispersion of said particulate materialand a sufficient amount of fiber to form a matrix of self-bondingfibers.